Non-destructive material testing for welds is well known in the art and depending on the particular welded materials and welding processes, various testing methods can be employed, including liquid penetrant testing, X-ray analysis, eddy current testing, and ultrasonic testing. While most of the currently known test methods provide reproducible and unambiguous results for many materials, ultrasonic testing (e.g., single-probe, TOFD, phased array, etc.) has proven to be particularly advantageous as the test results are typically immediately available using relatively simple equipment.
However, despite the versatility of ultrasonic testing, certain material flaws are often difficult to detect using such testing. For example, reheat cracks in heavy-walled hydrogen reactor vessels are often very small and clustered. To help improve detection of such flaws, various highly refined ultrasonic methods can be employed. For example, as described in U.S. Pat. No. 6,125,704, spectral response analysis of a suspect signal from a high-frequency angle beam transducer in pitch-catch mode is performed against a reference signal from a matching transducer that is operated in pulse-echo mode. In such methods, the signals from cracks formed by hydrogen attack will increase in amplitude with an increase in frequency, whereas no such dependence is evident from cracks due to welding defects. It should be noted that the cracks in Cr—V hydrogen reactors are considered reheat cracks. Although the cracking mechanism is not completely understood it is thought to be caused by a fracturing of the material due to stress and localized lowering of fracture toughness at a certain temperature range. Alternatively, as described in U.S. Pat. No. 5,404,754, amplitude-based and pattern-based backscatter signal analysis is used to identify and characterize defects due to hydrogen attack.
While such methods typically provide at least some improvement over conventional methods, various difficulties nevertheless remain. Among other things, the cracks caused by reheat cracking tend to be vertically oriented and transverse to the direction of the weld, and so render detection problematic where the detection employs conventional ultrasonic testing. Worse yet, as hydrogen tends to modify the fracture mechanical properties of the material, the standard flaw size requirements in the ASME Code often fail to provide adequate predictability of and confidence in proper service of such reactors.
TOFD ultrasonic testing in B-scan direction appears to provide the most sensitive and/or reliable manner of detection, however, various problems remain with such approach. For example, the sensitivity and calibration blocks pursuant to the ASME Code typically fail to replicate the response observed from actual reheat cracks for various reasons. Among other things, the fabricated flaws (side drilled holes) are not only too large to be comparable to reheat cracks, but due to the round acoustic interface also tend to reflect the energy back to the receiving probes rather than generating a weaker diffracted wave as would be the case with reheat cracks. Still further, the drilled holes in ASME blocks are arranged directly above each other, which results in a merging and confusion of the signals.
To overcome the difficulties associated with relatively large side drilled holes, smaller diameter side drilled holes may be used. However, such smaller round holes nevertheless tend to reflect the energy and generally provided unsatisfactory results. On the other hand, a series of flat bottom holes with progressively increasing distance could be used to discern lateral beam coverage. However, while the output in such tests will typically yield at least some information on lateral beam coverage, such output is limited to a single thickness plane (depth). Furthermore, replication of the response observed from reheat cracks is additionally complicated by the ASME code requirement to arrange the flaws parallel to the fusion line. In contrast, reheat cracks are typically transverse to the fusion line, which will invariably lead to a vastly different response in the TOFD output. On a finer note, the code also fails to require a sufficient number of flaws and/or to allow measurement of the lateral (width) area to thereby gain an improved understanding of the area of coverage enabled by each TOFD setup. Moreover, as the code is focused on conventional non-parallel scanning direction view, a potential lack of coverage of the entire weld width is not addressed where B-scan view in parallel scanning direction is employed. Such lack is particularly problematic where wide weld widths are tested.
Therefore, it should be readily apparent that while TOFD ultrasonic testing in B-scan direction may at least conceptually provide the most sensitive and/or reliable manner of detection for reheat crack detection, conventional sensitivity and qualification blocks dictated by the ASME Code fail to address critical issues, and such testing has not been implemented. Thus, there is still a need to provide improved devices and methods for TOFD sensitivity demonstration and calibration.